We use both.
We find that the voice recognition experience is almost always better with Alexa, and there are a few features, like being able to just say “Alexa, lights on” and having that apply to only the specific lights that we choose which is very helpful. Also, we find Alexa voice control of the television much better, because you can ask for a specific episode.
We use the HomePod mini primarily as the hub for our HomeKit system. There are a couple of voice inquiries that we ask it instead of Alexa. For example, for whatever reason, it is much better at answering the question of what the temperature is in a particular room. It’s also better at answering “what’s the low tonight?“ Alexa gives us the whole weather report, Siri just gives us the low.
So we generally look for devices which work with both HomeKit and Alexa, of which there are many these days.